Plastic laminate structures are used in various applications such as air frame or surface skin components of aircraft or space vehicles. Such structures typically are formed by laminating plastic composites in which reinforcing fibers are embedded in a plastic matrix material. Fibers useful in such composites include glass fibers such as E-glass or S-glass fibers, boron fibers, carbon-based fibers such as graphite fibers, and aramid fibers such as Kevlar 49. The plastic matrix material useful in the formation of such composite panels include polystryene, polyethylene, polypropylene, polysulfones, polyarylene sulfides, polyetherether ketone (PEEK) and polyetherimide (PEI). In high performance composite laminate structures graphite fibers are preferred since they impart an extremely high degree of rigidity to the panels, in particular, advanced high polymers such as PEEK, PEI, and the polyarylene sulfide are desirable as reinforced polymer matrices. These polymers, however, suffer the disadvantage of requiring relatively high temperatures before their thermoplastic melt or processing temperatures are reached.
In order to further strengthen the laminate panel structures, it is a conventional practice to incorporate stiffeners which extend from one side of the panel in an upstanding relationship. Various stiffeners which may be employed in composite panels are disclosed in "Glass Reinforced Plastics," Morgan, P., editor, John Wiley & Sons, New York, 3d ed., 1961, pp. 197-199, under the headings "Stiffeners" and "Sandwich Structure." As disclosed there, individual stiffeners may be formed separately and then bonded to the panel sheet or they may be molded in situ with the panel sheet. An alternative procedure, when employing laminates formed of two or more resin/fiber sheets, is to form a skin panel and a continuous hat stiffener separately and then bond these two components together through the use of adhesives or by induction welding of the plastic material in which the thermoplastic panels are heated locally to their thermoplastic melt points at a bonding pressure of perhaps 5-10 psi applied. Sandwich type structures of the type also described in Morgan may be employed by bonding yet another skin panel to the continuous hat stiffener section. Similarly, bonding can take place by means of adhesives or by induction welding.